全文获取类型
收费全文 | 16910篇 |
免费 | 2233篇 |
国内免费 | 8篇 |
出版年
2021年 | 210篇 |
2019年 | 172篇 |
2018年 | 203篇 |
2017年 | 179篇 |
2016年 | 303篇 |
2015年 | 485篇 |
2014年 | 554篇 |
2013年 | 659篇 |
2012年 | 775篇 |
2011年 | 841篇 |
2010年 | 543篇 |
2009年 | 521篇 |
2008年 | 677篇 |
2007年 | 666篇 |
2006年 | 577篇 |
2005年 | 610篇 |
2004年 | 624篇 |
2003年 | 574篇 |
2002年 | 545篇 |
2001年 | 513篇 |
2000年 | 562篇 |
1999年 | 469篇 |
1998年 | 213篇 |
1997年 | 213篇 |
1996年 | 194篇 |
1995年 | 198篇 |
1994年 | 199篇 |
1993年 | 184篇 |
1992年 | 399篇 |
1991年 | 389篇 |
1990年 | 382篇 |
1989年 | 338篇 |
1988年 | 326篇 |
1987年 | 301篇 |
1986年 | 250篇 |
1985年 | 274篇 |
1984年 | 232篇 |
1983年 | 217篇 |
1982年 | 172篇 |
1981年 | 170篇 |
1980年 | 164篇 |
1979年 | 240篇 |
1978年 | 205篇 |
1977年 | 187篇 |
1976年 | 183篇 |
1975年 | 180篇 |
1974年 | 174篇 |
1973年 | 196篇 |
1972年 | 182篇 |
1971年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
971.
Karen A. Johnson 《Annals of botany》2013,112(1):141-149
Background and Aims
Convergent floral traits hypothesized as attracting particular pollinators are known as pollination syndromes. Floral diversity suggests that the Australian epacrid flora may be adapted to pollinator type. Currently there are empirical data on the pollination systems for 87 species (approx. 15 % of Australian epacrids). This provides an opportunity to test for pollination syndromes and their important morphological traits in an iconic element of the Australian flora.Methods
Data on epacrid–pollinator relationships were obtained from published literature and field observation. A multivariate approach was used to test whether epacrid floral attributes related to pollinator profiles. Statistical classification was then used to rank floral attributes according to their predictive value. Data sets excluding mixed pollination systems were used to test the predictive power of statistical classification to identify pollination models.Key Results
Floral attributes are correlated with bird, fly and bee pollination. Using floral attributes identified as correlating with pollinator type, bird pollination is classified with 86 % accuracy, red flowers being the most important predictor. Fly and bee pollination are classified with 78 and 69 % accuracy, but have a lack of individually important floral predictors. Excluding mixed pollination systems improved the accuracy of the prediction of both bee and fly pollination systems.Conclusions
Although most epacrids have generalized pollination systems, a correlation between bird pollination and red, long-tubed epacrids is found. Statistical classification highlights the relative importance of each floral attribute in relation to pollinator type and proves useful in classifying epacrids to bird, fly and bee pollination systems. 相似文献972.
Brian M. Olson Laura E. Johnson Douglas G. McNeel 《Cancer immunology, immunotherapy : CII》2013,62(3):585-596
The androgen receptor (AR) plays an essential role in the development and progression of prostate cancer. However, while it has long been the primary molecular target of metastatic prostate cancer therapies, it has not been explored as an immunotherapeutic target. In particular, the AR ligand-binding domain (LBD) is a potentially attractive target, as it has an identical sequence among humans as well as among multiple species, providing a logical candidate for preclinical evaluation. In this report, we evaluated the immune and anti-tumor efficacy of a DNA vaccine targeting the AR LBD (pTVG-AR) in relevant rodent preclinical models. We found immunization of HHDII-DR1 mice, which express human HLA-A2 and HLA-DR1, with pTVG-AR augmented AR LBD HLA-A2-restricted peptide-specific, cytotoxic immune responses in vivo that could lyse human prostate cancer cells. Using an HLA-A2-expressing autochthonous model of prostate cancer, immunization with pTVG-AR augmented HLA-A2-restricted immune responses that could lyse syngeneic prostate tumor cells and led to a decrease in tumor burden and an increase in overall survival of tumor-bearing animals. Finally, immunization decreased prostate tumor growth in Copenhagen rats that was associated with a Th1-type immune response. These data show that the AR is as a prostate cancer immunological target antigen and that a DNA vaccine targeting the AR LBD is an attractive candidate for clinical evaluation. 相似文献
973.
Oliver J. D. Jewell Ryan L. Johnson Enrico Gennari Marthán N. Bester 《Environmental Biology of Fishes》2013,96(7):881-894
Previous work on white sharks indicate the species show seasonally limited movement patters, at certain aggregation sites small areas may play vital roles in the life history of a large amount of the population. Acoustic telemetry was used to estimate habitat use of white sharks, Carcharodon carcharias, while aggregating at Mossel Bay, South Africa. Total range of all shark tracks combined accumulated 782 h and covered an area of 93.5 km2 however, within this range, sharks were found to highly utilise a core habitat (50 % Kernel, K50) of just 1.05 km2 over a reef system adjacent to a river mouth. Individual tracks revealed additional core habitats, some of which were previously undocumented and one adjacent to a commercial harbor. Much was found to be dependent on the size of the shark, with larger sharks (>400 cm) occupying smaller activity areas than subadult (300–399 cm) and juvenile (<300 cm) conspecifics, while Index of Reuse (IOR) and Index of Shared Space (IOSS) were both found to increase with shark size. Such results provide evidence that larger white sharks are more selective in habitat use, which indicates they have greater experience within aggregation sites. Furthermore, the focused nature of foraging means spatially restricted management strategies would offer a powerful tool to aid enforcement of current protective legislation for the white shark in similar environments of limited resources and capacity. 相似文献
974.
Jan Fischer Jörg W. Schneider John-Paul M. Hodnett David K. Elliott Gary D. Johnson Silke Voigt 《Historical Biology》2013,25(6):710-727
δ18OP values and 87Sr/86Sr ratios were determined on disarticulated xenacanthiform, hybodontid and ctenacanthid shark tooth material from several Early Permian (Sakmarian–Kungurian) continental bone beds of northern Texas and southern Oklahoma as well as from the marine Middle Permian (Roadian) of northern Arizona. The δ18OP values derived from the teeth of bone beds are in the range of 17.6–23.5‰ VSMOW, and are mostly depleted in 18O by 0.5–5‰ relative to proposed coeval marine δ18OP values. This indicates an adaptation to freshwater habitats on the Early Permian coastal plain by several sharks. Distinctly higher δ18OP values from two bone beds are attributed to significant evaporative enrichment in 18O in flood plain ponds. 87Sr/86Sr ratios of around 0.71077 are notably more radiogenic than 87Sr/86Sr of contemporaneous seawater. In contrast, the isotopic composition of teeth from the marine Kaibab Formation is characterised by low δ18OP values in the range of 13.4–15.6‰ VSMOW while 87Sr/86Sr ratios of around 0.70821 are closer to the Roadian seawater value. The distinctly depleted δ18OP values cannot be readily explained by fluvially affected freshening in a nearshore marine environment, so a diagenetic alteration of the Kaibab material seems to be more likely, excluding it from further interpretation. 相似文献
975.
Zoe Barclay Louise Dickson Derek Robertson Melanie Johnson Pamela Holland Roberta Rosie Liting Sun Helen Jerina Eve Lutz Sue Fleetwood-Walker Rory Mitchell 《Cellular signalling》2013,25(4):814-821
The 5-HT2A receptor (5-HT2AR) is implicated in psychotropic changes within the central nervous system (CNS). A number of polymorphisms have been reported in the 5-HT2AR gene; one of these results in a non-synonymous change, H452Y, in the carboxy-terminal tail of the receptor protein. The minor allele (9% occurrence) has been statistically associated with CNS dysfunction such as impaired memory processing and resistance to neuroleptic treatment in schizophrenic patients. We investigated the impact of H452Y mutation of the 5-HT2AR expressed in COS7 cells on distinctly coupled intracellular signalling pathways from the receptor, focusing on the heterotrimeric G protein-independent phospholipase D (PLD) pathway, compared to the conventional Gq/11-linked phospholipase C (PLC) pathway. The H452Y mutation selectively attenuated PLD signalling, which as in the wild-type receptor, was mediated by a molecular complex involving PLD1 docked to the receptor's carboxy-terminal tail domain. Co-immunoprecipitation and GST-fusion protein experiments revealed that the H452Y mutation selectively reduced PLD1 binding to the receptor. Experiments with blocking peptides to mimic short sections of the 5-HT2AR tail sequence revealed that the peptide spanning residue 452 strongly reduced PLD but not PLC responses of the receptor. Similar observations were made when assessing both PLD responses and PLD-dependent cellular proliferation elicited by activation of 5-HT2ARs natively expressed in MCF-7 cells. Overall these findings indicate that the H452Y polymorphic variant of the 5-HT2AR displays selective disruption of its PLD signalling pathway. This may potentially play a role in the CNS dysfunction associated with the H452Y allele of the 5-HT2AR. 相似文献
976.
David W. Thieltges Per‐Arne Amundsen Ryan F. Hechinger Pieter T. J. Johnson Kevin D. Lafferty Kim N. Mouritsen Daniel L. Preston Karsten Reise C. Dieter Zander Robert Poulin 《Oikos》2013,122(10):1473-1482
While the recent inclusion of parasites into food‐web studies has highlighted the role of parasites as consumers, there is accumulating evidence that parasites can also serve as prey for predators. Here we investigated empirical patterns of predation on parasites and their relationships with parasite transmission in eight topological food webs representing marine and freshwater ecosystems. Within each food web, we examined links in the typical predator–prey sub web as well as the predator–parasite sub web, i.e. the quadrant of the food web indicating which predators eat parasites. Most predator– parasite links represented ‘concomitant predation’ (consumption and death of a parasite along with the prey/host; 58–72%), followed by ‘trophic transmission’ (predator feeds on infected prey and becomes infected; 8–32%) and predation on free‐living parasite life‐cycle stages (4–30%). Parasite life‐cycle stages had, on average, between 4.2 and 14.2 predators. Among the food webs, as predator richness increased, the number of links exploited by trophically transmitted parasites increased at about the same rate as did the number of links where these stages serve as prey. On the whole, our analyses suggest that predation on parasites has important consequences for both predators and parasites, and food web structure. Because our analysis is solely based on topological webs, determining the strength of these interactions is a promising avenue for future research. 相似文献
977.
Stuart W. Smith A. H. Jean Robertson Andrew A. Meharg Robin J. Pakeman David Johnson Sarah J. Woodin René van der Wal 《Plant and Soil》2013,369(1-2):509-513
Background and aims
Milling of plant and soil material in plastic tubes, such as microcentrifuge tubes, over-estimates carbon (C) and under-estimates nitrogen (N) concentrations due to the introduction of polypropylene into milled samples, as identified using Fourier-transform infra-red spectroscopy.Methods and results
This study compares C and N concentrations of roots and soil milled in microcentrifuge tubes versus stainless steel containers, demonstrating that a longer milling time, greater milling intensity, smaller sample size and inclusion of abrasive sample material all increase polypropylene contamination from plastic tubes leading to overestimation of C concentrations by up to 8 % (0.08 g?g?1).Conclusions
Erroneous estimations of C and N, and other analytes, must be assumed after milling in plastic tubes and milling methods should be adapted to minimise such error. 相似文献978.
Background
Recent studies have questioned the validity of the mutualism-parasitism continuum of mycorrhizal function. This paper re-evaluates the continuum model and analyzes these concerns.Scope and Conclusions
Three insights arise from this analysis. First, the continuum model defines mycorrhizal function as an emergent property of complex interactions. The model identifies resource trade and symbiotic control as key determinants of the costs and benefits of the symbiosis for plants and fungi, and the interaction of these factors with the environment ultimately controls mycorrhizal function. Second, analysis of carbon costs and phosphorus benefits is too narrow a focus to accurately predict mycorrhizal function. Analysis of plant and fungal fitness responses in ecologically and evolutionarily relevant systems are required to elucidate the full range of nutritional and non-nutritional factors embodied within mycorrhizal functioning. Finally, the definition of the term ‘parasitism’ has evolved. Some fields of science maintain the original definition of a nutritional relationship between host and parasite while other fields define it as a +/- fitness relationship. This has generated debate about whether the continuum of mycorrhizal functioning should properly be called a positive–negative response continuum or a mutualism-parasitism continuum. This controversy about semantics should be resolved, but it does not overturn the continuum concept. 相似文献979.
Daniel J. Salamango David A. Evans Mariju F. Baluyot Jackson N. Furlong Marc C. Johnson 《Journal of virology》2013,87(24):13900-13903
Fluorescent proteins are routinely employed as reporters in retroviral vectors. Here, we demonstrate that transduction with retroviral vectors carrying a tandem-dimer Tomato (TdTom) reporter produces two distinct fluorescent cell populations following template jumping due to a single nucleotide polymorphism between the first and second Tomato genes. Template jumping also occurs between repeated sequences in the Tomato and green fluorescent protein (GFP) genes. Thus, proper interpretation of the fluorescence intensity of transduced cells requires caution. 相似文献
980.
Julieta Manrique Michael Piatak William Lauer Welkin Johnson Keith Mansfield Jeffrey Lifson Ronald Desrosiers 《Journal of virology》2013,87(13):7246-7254
Vaccine/challenge experiments that utilize live attenuated strains of simian immunodeficiency virus (SIV) in monkeys may be useful for elucidating what is needed from a vaccine in order to achieve protective immunity. Derivatives of SIVmac239 and SIVmac239Δnef were constructed in which env sequences were replaced with those of the heterologous strain E543; these were then used in vaccine/challenge experiments. When challenge occurred at 22 weeks, 10 of 12 monkeys exhibited apparent sterilizing immunity despite a mismatch of Env sequences, compared to 12 of 12 monkeys with apparent sterilizing immunity when challenge virus was matched in its Env sequence. However, when challenge occurred at 6 weeks, 6 of 6 SIV239Δnef-immunized monkeys became superinfected by challenge virus mismatched in its Env sequence (SIV239/EnvE543). These results contrast markedly not only with the results of the week 22 challenge but also with the sterilizing immunity observed in 5 of 5 SIV239Δnef-immunized rhesus monkeys challenged at 5 weeks with SIV239, i.e., with no mismatch of Env sequences. We conclude from these studies that a mismatch of Env sequences in the challenge virus can have a dramatic effect on the extent of apparent sterilizing immunity when challenge occurs relatively early, 5 to 6 weeks after the nef-deleted SIV administration. However, by 22 weeks, mismatch of Env sequences has little or no influence on the degree of protection against challenge virus. Our findings suggest that anti-Env immune responses are a key component of the protective immunity elicited by live attenuated, nef-deleted SIV. 相似文献